The present invention relates to phenolic resins and more particularly to a catalyst system which imparts improved storage stability while retaining rapid cure characteristics.
Phenol or A-stage resins are well known to be prepared from a phenolic part and an aldehyde part which are catalytically reacted. A variety of ionizable catalytic agents are known in the art for the preparation of resole resins, including various alkali metals, alkaline earth metals, and amines. Residual ionizable catalyst permits subsequent heat cure of the resole resin, though at the expense of storage stability. A particular preferred class of resole resins are benzylic ether resins, such as described in U.S. Pat. Nos. 3,409,579 and 3,676,392. Benzylic ether resins can be formed by the reaction of a phenol and an aldehyde under substantially anhydrous conditions at temperatures below about 130.degree. C. In the presence of a catalytic concentration of a metal ion dissolved in the reaction medium. Benzylic ether resins also can be formed using catalysts, which the art has used in forming non-anhydrous resoles, by the aqueous reaction of a phenol and formaldehyde at low temperature, subsequently neutralizing the reaction medium to a pH of about 3.8-5.3 to form insoluble, non-ionizable salts, and then stripping the reaction medium under vacuum at temperatures ranging up to 130.degree. C. The insoluble, non-ionizable salts may be removed by filtration or other common means prior to the dehydration reaction in order to lower the residual ionizability even more. The preparation and characterization of some of these resins is disclosed in greater detail in U.S. Pat. No. 3,485,797. Such catalytic agents include sodium, zinc acetate, lead acetate, lithium naphthenate, lead napthanate, lead oxide, and the like. The benzylic ether resins as formed in the references above also are known as high ortho-ortho resins in that the resin is characterized by ortho-ortho linkages, compared to conventional resole resins, whether anhydrously formed or not, wherein ortho-para linkages predominate. Benzylic ether resins formed during low ionic dehydration possess advantages over conventional resoles in that they are slow to cure with heat. Consequently, they are very stable at room temperature due to the lack of ions present.
Again, the benzylic ether resins can be converted to a cured network in the presence of hydrogen ion-type catalysts, such as typified by strong inorganic and organic acids. Though such benzylic ether resins can be readily cured in the presence of strong acid catalyst, storage stability suffers by dint of such strong acid catalysts. Thus, there is a need in the resole art for maintaining, and even increasing the speed of cure of strong acid-catalyzed resole resins, while improving the shelf life thereof.